Confirm the mechanism of the complex strain response

Investigate whether pressure-induced reorientation of thermally induced martensite variants contributes to the complex strain response of powder-in-tube Heusler-alloy composites under uniaxial loading, including the nonlinear strain changes and the absence of a clearly resolved reverse transformation in the strain signal.

Background

The paper finds that macroscopic strain does not reliably track the transformed phase fraction in the powder-in-tube composite because the steel sheath, heterogeneous internal stresses, possible local plasticity, and martensite-variant processes contribute simultaneously to the measured response. In particular, pressure-induced reorientation of thermally induced martensite variants could change the macroscopic strain while leaving the magnetic properties of martensite largely unchanged. The authors present this as a possible explanation for the pronounced nonlinear strain changes during loading and for the absence of a clear reverse strain change upon heating, but do not establish the mechanism. They identify in-situ X-ray diffraction under load as the investigation required to confirm it.

References

In addition to this mechanical contribution, a pressure-induced reorientation of the thermally induced martensite variants might contribute to this complex strain behavior. This reorientation changes the macroscopic strain but leaves the magnetic properties of the martensite largely unchanged, explaining the lack of correlation with susceptibility and the deviation from pure bulk behavior . Specifically, the pronounced non-linear strain change observed within the initial and final \SI{50}{\mega\pascal} ranges of the loading cycles could correspond to this reorientation process.

— Powder-in-tube confinement as a design principle for fatigue-resistant caloric materials  (2610.08172 - Niehoff et al., 6 Oct 2026) in Section 3.4.3, “Complex Strain Response”